Subjecting materials to cryogenic and low-temperature levels affect many characteristics of various materials. While other uses and applications described herein exist and may be obvious from this disclosure, the apparatus and methods described here are directed to the following areas:
(1) Short and long term preservation of the quality, nutritional value, and palatability of food products, by both low-temperature and freeze-vacuum methods. PA1 (2) Creation of states of embrittlement or frangibility for rubber, plastics, metals, and other materials for subsequent processing to create fine-mesh particles, and PA1 (3) Separation of rubber or plastic base materials from other component parts, such as fibers and metals. For example, separation of rubber from used tires and conveyer belts.
As it relates to food products, the term "freezing" as used herein refers to the solidification or crystallization of water, moisture, and other fluid-like contents such as enzymatic and proteinaceous liquids by certain temperature- or heat-reduction methods prior to frozen storage. Freezing shall be sometimes referred to as ultrafreezing. Frozen storage refers to the holding of food and other products at constant sub-freezing temperatures. In the United States, frozen-storage temperatures are generally construed to be about 0.degree. F. or lower. In Europe, frozen-storage temperatures are considered to be about -10.degree. F. and lower.
The terms "cryogenic temperature", "cryogenic fluids" and "cryogenic coolants" are used in referring to temperatures, fluids and coolants below -250.degree. F. and 1-atm pressure respectively and coolant temperatures of -250.degree. F. or lower. "Low-temperature coolant" refers to those elements and compounds which exist in the liquid state at temperatures and pressures above -250.degree. F. and 1-atm pressure respectively but are capable of freezing or embrittlement of the materials discussed at these higher temperatures.
Generally, liquid nitrogen (LN.sub.2) is an excellent coolant for attaining cryogenic-temperature levels, but must be used efficiently to be cost effective. Liquid carbon-dioxide (CO.sub.2) spray, forming dry-ice snow and CO.sub.2 vapor at 1-atm pressure, is also used for freezing at -109.degree. F. and higher, especially in food applications. CO.sub.2, however, is a low-temperature coolant but not a cryogenic coolant since the minimum 1-atm temperatures attained (-109.degree. F.) are greater than -250&lt; F.
Many of the low-temperature or cryogenic-cooling or freezing methods are referred to as "IQF" freezing or "Instant Quick Freeze". This is somewhat of a misnomer since the implication is made that an item being treated is instantly reduced to the low-temperature or cryogenic-temperature levels. This may be the case for the extreme outer portions of the item but, as will be shown, the cooling or freezing times for the inner regions vary greatly depending on whether cryogenic-liquid, vapor-spray, or directional-cooling methods are used. Even with direct immersion of an item into a cryogenic fluid, such as LN.sub.2, some time is required for the inner regions of the item to reach the low or cryogenic temperature.
Some semi-solid materials, such as rubber and thermal-plastic products,do not have freezing- or fusion-temperature points as do liquids but undergo certain crystalline changes and become brittle or frangible at certain low temperatures especially in the cryogenic-temperature range. Since similar materials within an item tend to contract away from junctions with other materials when cooled to low temperatures, separation of metals and fibers from rubber in, for example, automobile tires is performed more easily when the tire is cooled to cryogenic-temperature levels.
For freezing food products, rubber, plastic, or other materials, the most commonly used methods are vapor-blast or spray freezing, low-temperature or cryogenic-liquid spraying, immersion into low-temperature or cryogenic-fluids, or combinations of these procedures. Low-temperature or cryogenic vapors or fluids introduced into a negative-pressure chamber containing the materials to be subjected to sub-freezing-temperature levels of the respective coolants is another method used for attaining very low temperature levels. Negative pressure or vacuum techniques are used for freeze drying many food products. This consists of freezing the products followed by subliming or vaporizing aqueous fluids of the frozen portions in a vacuum to facilitate even longer storage times and reduced shipping weights.
Vapor-spray freezing generally entails subjecting articles or products to vapors emanating from perforated tubing above the products as they are conveyed through a tunnel-like apparatus. This practice is widespread and used commercially to freeze food for long-term frozen storage and to embrittle rubber, plastics, and other products. Generally, the tunnel-vapor process utilizes vapors and carbon-dioxide snow emanating from liquid carbon dioxide (LCO.sub.2) or liquid-nitrogen (LN.sub.2) storage tanks through transfer lines to perforated tubing running along the tops, sides and/or bottoms of insulated tunnels. The vapors may be further circulated for more efficient cooling by fans situated at the ends or inside of the tunnels. Expended vapors may also be used for pre-freeze cooling of the products prior to entering the tunnel. Many tunnel systems utilize urethane or styrofoam insulation around the tunnel walls which allows greater coolant losses than would be incurred by other methods of insulation.
Vapor-blast or spray methods may also be used in conjunction with liquid-spray application as the product is conveyed along the length of the tunnel. This ensures more rapid formation of frozen crusts around the outer area of the products if used at the entry portion of the tunnel and more thorough freezing of the product if used near the exit end of the tunnel.
The liquid spray of LN.sub.2 has greater heat-absorption capabilities than the vapor spray. Liquid-spray procedures may also be used along the entire length of the tunnel if fracturing, rupturing or breakage pose no difficulties such as when treating rubber and plastic for embrittlement or for separation of rubber from metal or rubber from fiber materials. Fracturing, rupturing, or breakage may occur on food or other materials when freezing too rapidly occurs on the outside of the articles due to the different expansion/contraction rates of the frozen outer crusts and unfrozen or warmer inner portions of the products.
In conjunction with or separate from the vapor-spray and/or liquid-spray tunnel methods, some freezing techniques employ liquid-immersion steps or mechanical refrigeration techniques utilizing such coolants as brine or sodium chloride solutions. Liquid-nitrogen immersion may also be performed on food products to attain partial freezing or ice crusts prior to entering the vapor-spray or liquid-spray section of the tunnel to complete the freezing process at lower temperatures prior to frozen storage. Higher-than-cryogenic-temperatures utilizing liquid CO.sub.2 as the immersion medium is used in some commercial methods.
Some frozen storage facilities may be also be used for freezing chambers. This generally entails expensive insulation costs. The applicant has observed one facility where crawfish and blue crab were placed, after being cooked and chilled to 35.degree. F., into a storage facility maintained at -5.degree. F. by mechanical refrigeration prior to being filled with the boxed products. Carbon-dioxide vapor and dry-ice snow were then sprayed from tubes around the top of the freezing room until the temperature of the highest portion of the stacked products reached -40.degree. F. over a period of about 6 hours. After a holding time of about 20 hours, the equilibrium temperature reached -10.degree. F. or colder, at which time the boxed products were loaded into and shipped in frozen-storage containers maintained at below -10.degree. F. Although described by the operator as such, this procedure does not suffice for long-term frozen storage (6 months or more) because of the relatively high pre-storage freezing temperatures and relatively slow freezing rates used.
Another commercial method commonly used is referred to as the spiral or spiral-belt freezing system. This consists generally of a spiral conveyor-belt system running from the bottom, around, and through the top of a vertical, insulated chamber. Liquid nitrogen or carbon-dioxide vapor or dry-ice snow is injected into the top or bottom of the chamber until desired temperature levels are attained. This system requires less space than the previously mentioned tunnel systems which generally are 20 to 60 feet in length or even longer in some large-volume operations.
Still another freezing technique for food products is that of the freeze-plate or contact method. This consists of placing the products onto and between horizontal plates or adjacent to vertical plates. Refrigerants are circulated through channels inside the plates while some pressure is maintained on the products to sustain close contact between the product and the plates. This method again primarily utilizes commercial refrigerants such as brine, but longer freezing times are required than for the previously discussed procedures.
Most of the freezing procedures described generally involve isothermal freezing or heat absorption, especially in the tunnel and the spiral systems, or long freezing times as in the plate-or contact-freezing procedures. In the tunnel-freezing methods, which are becoming more prevalent both for food products and for embrittlement or rubber, plastic, metal, and other materials only a relatively small percentage of the vapor or liquid spray is in contact with the surface area of the product. This requires large amounts of coolant especially for rapid-temperature reduction. Greater amounts of coolant, which is not fully utilized, are also required for attaining ultimate low temperatures of the product which, if used efficiently, would be that of the coolant. The tunnel volume must also be sufficiently large to accommodate maximum product volume. Since the tunnel volume must essentially be inundated with vapor or liquid spray, less-than-maximum product amounts are frozen less efficiently. Some processing facilities store products until sufficiently large volumes are available for efficient freezing operations. This can be very detrimental to the quality of the frozen product, especially for such products as blue crab or crawfish, since both have rapid deterioration rates if not rapidly frozen to low-temperature levels.
As discussed earlier, freezing or cooling too rapidly, which can be done by immersion freezing or injecting large amounts of vapor or liquid spray, may also result in fracturing, rupturing, or breakage of many food products because the freezing temperatures for the outer portion are colder than those of the inner portion resulting in large internal stresses which can ruin the product. This can be overcome by initially forming frozen crusts or allowing only a certain portion of the product to freeze; the remaining portion of the product is therefore frozen at higher temperatures as equilibrium temperatures between that of the colder crusts and that of the higher-temperature inner portions are attained. This technique is being used with some success, but, obviously, product size variations may require various crust formation times as well as temperature equilibration times.
The problems just described for some food products are desirous for materials such as rubber, plastics, and other non-food products which are frozen for embrittlement or frangibility purposes prior to subsequent comminution. However, immersion freezing does not lend itself to viable conveyor methods. Conveying rubber through a liquid coolant is a tedious procedure. The expense of tunnel injection of large amounts of vapor or spray onto reclamation products, for example, rubber from used automobile and truck tires, generally low-cost materials, can be cost prohibitive if efficient methods are not used. Also, sufficient quantities of product must be at hand to fully optimize large-volume tunnel designs.
Bacterial growth may also result from damage created by large-crystal formation when freezing food products. Bacteria counts are generally higher for items which have been frozen at higher freezing temperatures with lower temperature-reduction rates, and bacteria are more easily propagated because of cell-wall damage by slower freezing rates of saline and enzymatic fluids within some food products.
As freezing temperatures are reduced below 32.degree. F., for many seafood and meat products and crustaceans, such as crawfish and blue crab, purer water in the tissues freezes out into large ice crystals if slow temperature-reduction rates are used. The remaining salt and enzymatic fluids may not freeze until temperature levels below 0.degree. F. are attained. These salts and enzymes are highly corrosive to meat-and fat-cell walls, and the formation of large frozen ice crystals may further damage cell walls. Textural damage and reduced palatability may be significant due to these factors.
Generally, for all food products except those that are freeze dried, drip loss or loss of product fluids or juices is another factor that can be correlated to large-cell ice crystal formation during freezing. This is a factor that is measured routinely on commercial products that are frozen and stored for long periods. Generally, drip losses in non-acceptable levels are observed for those products which have been frozen at warmer temperatures and/or slower rates prior to long-term frozen storage. For these reasons, low temperatures should be attained as quickly as possible. Frozen-storage temperatures should also be as low as economically feasible, preferably -10.degree. F. or lower, to minimize activity of enzymatic and other fluids. Generally, the higher the water, fat, and enzyme content, the more critical the attainment of low-temperature freezing levels preferably to below -250.degree. F. or cryogenic levels, and maintenance of low frozen-storage temperatures, preferably -10.degree. F. and below.